Abstract Graves' disease (GD) is a common autoimmune syndrome affecting the thyroid and orbit. The orbital manifestations, termed thyroid associated ophthalmopathy (TAO), are heterogeneous, but can include strabismus and loss of vision from expansion of the extraocular muscles and orbital fat. Currently, there are no therapies shown to prevent, slow or reverse the progressive and permanent effects of TAO. Furthermore, there are no surrogate markers of disease activity or severity to guide treatment. The mechanisms of immune infiltration of TAO are unclear, but fibroblasts are proposed as the orbital cell targets. Over-representation of cytokines also appears to play a critical role in both the inflammatory and fibrotic manifestations of disease. Our long-term goal is to understand the unifying mechanisms underlying the thyroidal and orbital involvement in GD. These insights should provide biomarkers for assessment of disease activity and promote the development of targeted treatment. We have recently implicated bone marrow-derived fibroblast precursors, called fibrocytes in TAO. Specifically, we identified increased levels of fibrocytes in the peripheral blood and orbital tissue of patients with TAO compared to healthy controls. We also demonstrate that these cells are phenotypically and functionally similar to TAO fibroblasts and constitutively express CD40. Moreover, fibrocyte activation via CD40 elicits several cytokines which bear pathologic relevance to TAO. We hypothesize that highly abundant circulating fibrocytes preferentially infiltrate the TAO orbital tissue and through activation of CD40, mediate inflammation and fibrosis through local production of cytokines. We propose to identify the clinical parameters associated with increased fibrocyte levels from TAO patients. Based upon our preliminary data, we have identified that TAO patients with severe disease have increased fibrocytes levels compared to patients with stable TAO. Our working hypothesis is that fibrocyte level is altered during the disease process and/or treatment. We also propose to determine the mechanism and role of CD40-mediated fibrocyte expression of select cytokines implicated in TAO. We have demonstrated CD40 expression by fibrocytes for the first time in this proposal, therefore the signaling mechanisms are yet unexplored. However, we hypothesize that CD40 activation of fibrocytes is mediated by canonical signal transduction pathways. The studies proposed will identify the clinical manifestations associated with increased fibrocyte levels and the CD40-mediated mechanisms of fibrocyte cytokine production. We anticipate these findings will lead to biomarker development and the introduction of novel therapies for TAO.